Optical Wavelength Division Multiplexing (“WDM”) has become a standard technology for fiber optic communication systems for the transmission of voice, data, the Internet, etc. WDM systems employ signals consisting of a number of different, unique wavelengths or channels, to transmit information. Each wavelength channel is modulated by a data signal, typically in the form of a stream of bits, which encode the voice or Internet traffic. As a result, a significant number of data signals may be transmitted simultaneously over a single optical fiber using WDM technology.
Despite the substantially higher fiber bandwidth utilization provided by WDM technology, multiplexing and demultiplexing create a number of serious problems that must be overcome, such as cross-talk, equalization, chromatic dispersion, network management, and routing of the information signals, for such systems to be commercially viable. Testing and troubleshooting problems are also greatly complicated by the additional components and complexity of a WDM network. Without additional testing tools, network maintenance is very difficult, resulting in significant time and effort expended to install and maintain a WDM network.
Multiplexing involves the process of combining multiple signals (each signal on its own wavelength) into a single multiple wavelength WDM signal. De-multiplexing is the opposite process in which each single wavelength is extracted and decomposed from the multiple wavelength signal. Each signal is thus reconstructed to match the original information signal before multiplexing.
Each wavelength channel has the capability to carry several gigabits of binary data per second. This is also referred to as the modulation rate. As the modulation rate is increased, more data can be carried, since each bit transmitted causes the carrier signal to be modulated. The modulation rate is currently defined by industry standards, SONET (“Synchronous Optical NETwork”) developed by the American National Standards Institute (“ANSI”) in the United States and used in North America, and SDH (“Synchronous Digital Hierarchy”) developed by the International Telecommunication Union (“ITU”) and used throughout most of the rest of the world.
Currently, nearly all information transmitted over fiber, whether voice, data, Internet, or e-mail, is done using the SONET/SDH standard. However, other standards for transmission of high data rates are emerging, such as Gigabit Ethernet and 10 Gigabit Ethernet. The present invention applies to the transmission of SONET/SDH, Ethernet, or other standards or proprietary protocols that may emerge in the future.
Understandably, as with many standards, use of the SONET/SDH standard has become not only typical but effectively required, because both the network transmitter and the network receiver must operate under identical standards so that the receiver can decipher the information sent by the transmitter. By using equipment that conforms to the standards, carriers (companies that build and operate networks) may then mix equipment from different vendors for their networks.
One reason SONET has become so successful is that it was designed so that the integrity of the data stream can be verified, even when live traffic is being transmitted. There are a number of established test equipment vendors building test-sets for analyzing SONET and SDH. Precise measurements of the error performance of the bit stream can be made. Equivalent test equipment can be expected in the future for Ethernet and other standards that may emerge, since test and verification is required to operate a successful network.
Current technology allows for a modulation rate of between 51 Megabits per second (“Mbps”) and 10 Gigabits per second (“Gbps”). An increase in the modulation rate results in a spectrally wider channel signal. Consequently, the wider signal and narrower spacing between channels mean that the signals are closer together, and thus harder to separate. As a result, data loss and distortion, such as crosstalk from adjacent signals, may occur.
As greater and greater amounts of data needed to be transmitted, further technological improvements led to the deployment of an improved, higher capacity protocol called dense wavelength division multiplexing (“DWDM”), which allows even more data streams (channels) to be transmitted over a single strand of fiber.
For data quality and system performance analysis, there are a number of very well established test equipment vendors who manufacture test-sets and testing mechanisms for analyzing SONET/SDH networks, as previously indicated. There are also established methods to look at the WDM signal, particularly on a physical layer level. For example, by looking at the WDM spectrum, various anomalies can be determined, such as cross-talk, correct channel wavelength and power levels, channel power equalization, and background noise levels.
Unfortunately, there is no effective way to combine the WDM and the SONET analytical techniques. Further, in the typical situation where there is a particular SONET/SDH (or other) signal of interest in a WDM on a fiber, it is difficult to extract and analyze that individual SONET/SDH (or other) signal.
The difficulty in using contemporary SONET/SDH testing equipment to monitor just one single WDM wavelength resides in the SONET/SDH receivers, which are designed to receive a single SONET/SDH data stream. The receivers therefore cannot be used directly to analyze WDM transmissions. If a WDM signal is directly inputted into a SONET/SDH analyzer, the test set will be unable to extract all the different individual data signals. The output will be meaningless and the test useless. It is not possible to extract each individual wavelength so that each can be analyzed individually.
Each single wavelength or carrier channel may carry upwards of 10 gigabits of data per second, each made up of thousands of tributary channels, called T1 lines. Carriers and equipment vendors find it necessary to be able to analyze each T1 and to verify each for quality. The scale of the challenge is daunting: to monitor all these information channels within the single SONET/SDH carrier channel wavelength, and then to multiply that by up to 81 or more different wavelengths that are possible in a WDM network.
The monitoring and testing generally falls into two analysis categories. One category is analyzing for defects on networks carrying live traffic, also referred to as “in service” testing. If a problem occurs, a network element will signal an alarm, which is transmitted inside the data overhead so that the remainder of the network (and the network operators) can identify the problem and react to it. It is also possible to detect transmission errors since parity checking is usually specified in the standards. By looking at the overhead for alarms and defects, the health and the quality of the circuit can be determined.
The other analysis category is bit error rate (“BER”) testing. This is “out-of-service” monitoring that is performed on a line when it is out of service. In that state, there is nothing on the line except what the monitoring tester puts on it. Typically, a pseudo-random test pattern, such as 223−1, is utilized to send a number of bits in pseudo-random sequence from one end of the line to the other. It is then possible to identify if any of those bits is received in error at the other end. Note that this out-of-service bit error rate testing needs to be done for every one of those hundreds or thousands of information channels across the plurality of carrier channel wavelengths.
A long felt need therefore remains for a method and apparatus for testing SONET/SDH signals on a WDM network, in which established in-service and out-of-service SONET/SDH testing protocols and capabilities can be advantageously employed in an accurate, rapid, effective, timely, and cost effective manner. A need also remains for a testing capability that is automatic and can therefore execute when and as needed, regardless of operator availability, and is not subject to possible operator error. The same need will exist for the testing of emerging standards that will be carried in WDM channels.
Solutions to problems of this sort have been long sought, but have long eluded those skilled in the art.